drm/msm/dsi-staging: fix default value for mdp transfer time

Fix the default value assigned to mdp transfer time when
corresponding dt entry of the panel binded is not available.

Change-Id: I3f486709b0cc9b7ccee446ebde640a2f8f607ec7
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>